The isomers of xylene find wide and varied application. For example, meta-xylene (m-xylene) is used in the manufacture of dyes, and ortho-xylene (o-xylene) is used as a feedstock for producing phthalic anhydride, which finds use in the manufacture of plasticizers. However, currently the most valuable of the xylene isomers is para-xylene (p-xylene), since p-xylene is a feedstock for terephthalic acid, which in turn is used in the manufacture of polyester fibers and films.
The majority of p-xylene produced today is derived from crude oil via reforming of the naphtha portion of the crude into a mixture of benzene, toluene, and xylenes (BTX) and heavier aromatics. These aromatics then undergo a variety of reactions, such as transalkylation, disproportionation, and xylene isomerization, to increase the concentration of p-xylene. The current commercial process also requires extraction of aromatics from non-aromatics and separation of p-xylene from a mixture of xylene isomers via crystallization or molecular sieve adsorption. The overall process is therefore complex. Moreover, as crude oil prices rise, so does the feed stock price for p-xylene production via the current commercial routes. Recently, as crude prices have risen, the prices of coal and natural gas having fallen making syngas derived from these sources cheaper on a carbon or energy equivalent basis and a potentially attractive feed for the production of basic chemicals, such as p-xylene.
The conversion of syngas to olefins and paraffins has been widely practiced for many years via the Fischer-Tropsch process and indirectly via the methanol to olefins (MTO) process. However, both of these syngas conversion routes produce only small amounts of aromatics and there is therefore a need for an improved route for converting syngas to aromatics, and particularly, p-xylene.
In a paper entitled “Direct Conversion of Syngas into Aromatics over Bifunctional Fe/MnO—ZnZSM-5 Catalyst”, Chinese Journal of Catalysis, Volume 23, No. 4 July, 2002, Wang Desheng et al. report that syngas can be converted into aromatics at high yield using a bifunctional catalyst comprising Fe/MnO mixed with Zn-ZSM-5 containing up to 7 wt % Zn under conditions including a temperature of 517° F. (270° C.), a pressure of 160 psig, and a hydrogen to carbon monoxide molar ratio of 2:1. However, the aromatic product slate obtained in the process of Wang et al. is composed mainly of benzene and toluene, rather than the more desirable p-xylene. Further, the preferred operating conditions of the Fischer-Tropsch process and aromatization process differ, limiting the conversion and selectivity to aromatics.
Another method for the conversion of syngas to higher hydrocarbons is isosynthesis, in which syngas is converted to C4 hydrocarbons. Because the isosynthesis reaction occurs in the same preferred temperature range as the aromatization reaction, there is a desire to combine isosynthesis with aromatization to provide a process of converting syngas to aromatics in which the yield of xylene isomers, and in particular p-xylene, is improved.